Total DOS Research Articles

Thermal expansion of the perovskite-type NdGaO3

The structure of NdGaO3 was investigated by means of powder diffraction using synchrotron radiation in the temperature range 12–1173K. The NdGaO3 structure can be assigned to the GdFeO3-type (space group Pbnm) in the whole temperature range investigated. The thermal behaviour of the cell parameters was measured and found to be highly anisotropic. NdGaO3 shows practically the same thermal expansion along [100] and [001] directions, whereas the expansion along [010] is about 40% lower. In order to calculate the NdGaO3 thermal expansion three independent simulation techniques were applied: lattice dynamics, molecular dynamics and the Debye model. Using the GULP code and interatomic potentials described in the Buckingham form the Nd–O and Ga–O interactions have been derived. The total phonon DOS, its projections onto atomic species, the heat capacity Cv, the Grüneisen parameter γ and the thermal expansion coefficient α were determined using quasiharmonic lattice dynamics. Molecular dynamic simulations were applied to model the thermal expansion at different pressures and temperatures. The structure, its elastic properties and melting temperature are close to the experimentally observed values, which indicate the correct choice of the interatomic potential form and parameters. Comparison between the total DOS and its projections with the Debye DOS and Grüneisen parameter values calculated for different approaches allows to conclude that NdGaO3 cannot be considered as a good Debye-like solid.

Phonon assignments in GaN bulk

The measured phonon-density of states of bulk GaN by time-of-flight neutron spectroscopy has been recently reported by Nipko et al. [1]. The authors have also calculated the true partial and total DOS as well as the phonon dispersion curves along major symmetry directions in the Brillouin zone. However, the group-theoretical phonon assignments have not been provided. Based on calculated symmetry allowed modes spanned by displacement representation and on the derived connectivity relations along the major directions in the Brillouin zone we have assigned Nipko's phonon dispersion curves to irreducible representations (species) of the C 4 6c (P6 3 mc) space group of GaN.

The study of the atomic and electronic structures of liquid copper

In the work, the investigation of the atomic and electronic structure of liquid copper has been conducted using the molecular dynamics (MD) modeling method and the method of band calculation tight-binding linear muffin-tin-orbital atomic sphere-approximation (TB-LMTO-ASA) method. The experimental study of the electronic structure has been performed with the use of the 30-cm X-ray electronic magnetic spectrometer. The theoretical and experimental results of studying the electronic structure of liquid copper are in a good agreement. In the liquid state, a strong hybridization of d-electrons with p-electrons of copper atoms is observed which influences the shape of the valence bands and the curves of the total and d-partial DOS.

The crystal structure and thermal expansion of the perovskite-typeNd0.75Sm0.25GaO3: powder diffraction and lattice dynamical studies

The structure of Nd0.75Sm0.25GaO3 was studied by high-resolution powder diffraction methods using conventionalx-ray and synchrotron radiation in the temperature range 85–1173 K. TheGdFeO3 structure typewas confirmed for Nd0.75Sm0.25GaO3 in the temperature region investigated and no structural transitions were observed. The cellparameters show a monotonic and anisotropic increase with temperature. The interatomicpotential was fitted using the GULP code. Using this potential, a self-consistentapproximation following the Debye model was constructed from the elastic constants of thecrystals. The total phonon DOS, its projections onto atomic species, heat capacityCv, Grüneisen parameterγ and thermalexpansion coefficient α were considered in the framework of quasiharmonic lattice dynamics and the Debye model.The shape of the phonon DOS calculated from lattice dynamics differs significantly fromthe respected Debye DOS. The rare earth, gallium and oxygen atoms dominate in differentfrequency regions of the phonon spectrum. The heat capacity is well reproduced by theDebye model below 100 K, where acoustic phonons play an important role and above800 K when the classical limit is reached. Predicted values of Grüneisen parameterand thermal expansion coefficients in the frame of the Debye model are too low. Therefore, the thermal properties ofNd0.75Sm0.25GaO3 cannot be explained by acoustic phonons only and hence,Nd0.75Sm0.25GaO3 cannot be described perfectly as a Debye-like solid with respect to its thermodynamicproperties.

Electronic structure of magnesium diboride and related compounds

AbstractThe electronic structure of AlB2‐type diborides and related compounds has been investigated in first‐principles calculations with the molecular cluster discrete variational method. For MgB2 was studied the effect of the lattice relaxation on the total density of states at the Fermi energy (N(ϵF)). The results indicated that a contraction of about 2% in the lattice spacings a and c can lead to a slight increase of N(ϵF) for boron. In the MB2 diborides, M = Al, Ti, V, Cr, Zr, Nb, Mo and Ta, the largest contributions to N(ϵF) is observed for Cr, Mo and Nb. TiB2 possess the highest chemical stability in the series. The electronic specific heat coefficient γ also is calculated for the diborides. The method is employed to obtain the partial B2p contribution to the total DOS at the Fermi level with the introduction of a monolayer of solute atoms as a substitution for Mg atoms of Na, Al, Ca, Ti, V, Cr, Zr, Nb, Mo and Ta in layered superstructures … /M/B2/Mg/B2/ … . A stronger covalent bonding between boron atoms is identified in these cases. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extended Huckel tight-binding calculations of the electronic structure ofYbFe4Sb12,UFe4P12,andThFe4P12

Calculation of the band structure, total and projected density of states, crystal orbital population analysis (COOP), and Mulliken population analysis were performed for the filled skutterudites ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12},$ ${\mathrm{UFe}}_{4}{\mathrm{P}}_{12},$ and ${\mathrm{ThFe}}_{4}{\mathrm{P}}_{12}.$ The calculated energy bands depict a semimetal behavior for ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ and ${\mathrm{UFe}}_{4}{\mathrm{P}}_{12},$ and metallic behavior for ${\mathrm{ThFe}}_{4}{\mathrm{P}}_{12}.$ Furthermore, the contributions from each orbital to the total DOS for each compound corroborate these findings. The bonding strength was derived from the COOP analysis between different pairs of atoms, considering nearest neighbor distances between 3.40 and 6.47 \AA{} for ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12},$ 2.91 and 6.47 \AA{} for ${\mathrm{UFe}}_{4}{\mathrm{P}}_{12},$ and 2.48 and 5.51 \AA{} for ${\mathrm{ThFe}}_{4}{\mathrm{P}}_{12}.$ Mulliken population analysis suggests ionic behavior for these compounds.

Electronic structure and chemical bond of titanium diboride

Titanium diboride was calculated by the density function and discrete variational (DFT-DVM) method to study the relation between structure and properties. Titanium and its first-nearest boron atoms form a strong covalent bond, so TiB2 has high melting point, hardness and chemical stability. Titanium atom releases two electrons to form Ti2+ ions, and a boron atom gets one electron to come into B− ion. B− takes the sp2 hybrid and forms σ bonds to link other boron atoms in the same layer. The other one 2pz orbital of every B− ion in the same layer interacts each other to form the π molecular orbital, so TiB2 has fine electrical property. The calculated density of state is close to the result of XPS experiment of TiB2. Mainly Ti3d and B2p atomic orbitals contribute the total DOS near the Fermi level.

Phonon- and phason-dynamics in Al–Cu–Fe quasicrystals

The lattice dynamics of quasicrystals includes local phason jumps as well as phonons. Phason dynamics is important for the understanding of both the structure and atomic motion in quasicrystals, leading to short-ranged atomic motion not involving vacancies in addition to diffusion. We have studied the phason and phonon dynamics of icosahedral i-Al62Cu25.5Fe12.5. Quasielastic Mossbauer spectroscopy (QMS) was used to probe the iron phason dynamics. Inelastic nuclear-resonant absorption (INA) of synchrotron radiation and inelastic neutron scattering (INS) were used to study the iron-partial as well as the total vibrational DOS (VDOS). We find from preliminary QMS studies that iron atoms jump on a time scale about two orders of magnitude slower than that found for copper. The EFG shows an abrupt change in slope at ca. 825 K which may be related to a transition from simple (isolated) to more complicated (co-operative) phason jumps. From INA we find that the iron-partial VDOS differs radically from that of the total (neutron-weighted) generalised VDOS measured by INS. Both these properties are related to the specific local environments of Fe and Cu in i-AlCuFe.

Plasmon DOS in layered systems: two layers per unit cell

In this paper we present the partial and total DOS of the plasmons in a layered electron gas system consisting of two layers per unit cell using the Random Phase Approximation. The description of a lattice in terms of conducting bilayers surrounded by a non-conducting medium can be seen as a first approximation to oxide superconductors like Bi2212 and Tl2212. The relevancy of the partial density of states for experimental probes like reflection EELS will be briefly discussed. Preceding the exact description of the partial DOS we will give a short summary of the derivation of the plasmon dispersion in a two layer/unit cell system [A. Griffin, A.J. Pindor, Phys. Rev. B 39 (1989) 11 503] and describe the two different branches that emerge. Finally, the total DOS is calculated numerically and is compared to the one layer/unit cell case.

Low-temperature specific heat of near-equiatomic Ni-rich B19′-TiNi-alloys

The total DOS has been calculated for B19′-TiNi-alloys using three compositions (50.0, 51.6 and 53.1 at.% Ni) by means of the linearized muffin-tin-orbital (LMTO) method. No drastic increase in the Fermi-level DOS has been found at 51.6 at.% Ni (with respect to the composition of 50.0 at.% Ni). There is a slight increase at 53.1 at.% Ni. The electron–phonon mass enhancement factor (λ ep ) has in this case only a small contribution to the electronic coefficient of the low-temperature specific heat (γ). According to our calculations, it is not the composition in itself which can cause the large experimental value of γ at 51.5 at.% Ni. Our results agree with the view that it is not the high Fermi-level DOS but an anomaly in the electron–phonon system which is responsible for the composition-dependent variation of the martenstitic transition temperature ( T Ms ). It is probable that the large experimental value of γ at 51.5 at.% Ni is caused by impurities: by precipitates or impurity phases.

Phonon excitations and related thermal properties of aluminum nitride

The phonon density of states of aluminum nitride was determined by time-of-flight neutron spectroscopy using a polycrystalline sample. The observed phonon excitation spectrum consists of a broadband centered at about 35 meV, a small gap in the 75--80 meV region, and two sharp bands at approximately 85 and 92 meV. A rigid-ion model was applied to the interpretation of the data. After optimization, the model provided a satisfactory description of the neutron results as well as the Raman and IR data, sound-velocity measurements, and the lattice specific heat reported in the literature. The partial and total DOS and the phonon-dispersion curves along major symmetry directions of the Brillouin zone were calculated, and the contribution of phonons to the Debye behavior of the low-temperature thermal conductivity was discussed.

Crystal orbital scheme for Si 3N 4

The crystal orbital (CO) scheme is constructed for β-Si 3N 4 to display the bonding in the solid compound. The electronic band structure is obtained within the extended Hückel tight-binding approach for periodical systems. For the purpose of comparison features of the electronic structure are contrasted for a series of SiC, Si 3N 4, and SiO 2. Partial DOS are displayed to highlight orbital interactions. Combined with bonding properties of states they are used to assign simplified interaction diagram consisting of atomic energy levels, total DOS distribution and interaction lines. The interaction scheme provides well-arranged synoptical picture showing fundamental factors responsible for the final distribution of energy states. Via the CO scheme the bonding in β-Si 3N 4 is displayed in the form understandable to people with no knowledge of quantum mechanics.

Low-frequency quantum transport in a three-probe mesoscopic conductor

We investigate the low-frequency quantum transport properties of a three-probe mesoscopic conductor. The static transmission coefficients and emittance matrix of the system were computed by explicitly evaluating the various partial density of states (PDOS). We studied the finite-size effect of the scattering volume on the global PDOS. By increasing the scattering volume we observed a gradual improvement in the agreement of the total DOS as computed externally or locally. Our numerical data permit a particular fitting form of the finite-size effect. Finally, we propose a method to solve the finite-size problem.

Crystal orbital schemes for solids. Titanium and vanadium monocarbides and mononitrides

Crystal orbital (CO) schemes are proposed for presenting the chemical bonding in solids. The CO scheme, based on the total DOS curve, shows interactions controlling main features of the electronic structure of a solid compound in a compact form. Its construction must be preceded by a detailed analysis of orbital interactions and bonding proclivities of states resulting from the band structure calculation. The schemes are demonstated for simple cubic transition metal carbides and nitrides. Although the chemical bonding in these simple compounds is well understood the analysis not only shows new features not described so far, but easily proves erroneous conclusions of several recent interpretations.

Magnetic moment of a Co atom in the pseudobinary Co(Ti1−xAlx) Heusler alloys

Abstract The electronic densities of states of the disordered Co(Ti 1− x Al x ) alloys are calculated by using the TB LMTO-CPA method. We observe the change of the shape of the total DOS with the increase of the concentration of Al. The value of the magnetic moment on the Co atom increases from zero x = 0 to 0.30 μ B for x = 0.5 and then it decreases again to zero for x = 1.

Theoretical study of CN chemisorbed on the Pd(111) surface

The chemisorption of CN on the Pd(111) surface has been studied by the DV-Xα and ASED-MO methods using cluster models. The binding energies and equilibrium geometries for several adsorption sites have been determined by the ASED-MO method. The results show that the center superposition site (hcp) with the CN axis parallel to the surface and to the [11 2 ] direction is the most stable site and the optimized height is equal to 1.51 Å. We also calculated the electronic structures with the DV-Xα method. In the total DOS curves for the equilibrium adsorption geometry, four peaks located at 8.77, 7.25, 6.02 and 5.09 eV below E F are found. They are determined to be the 4σ(a′), 1π(a′), 1π(a″ and 5σ(a′) levels respectively. The total charge transferred from the substrate to the CN is about 0.51 electrons, which is mainly in to the 5σ orbital.

Theoretical study of CN chemisorbed on the Pd(111) surface

The chemisorption of CN on the Pd(111) surface has been studied by the DV-Xα and ASED-MO methods using cluster models. The binding energies and equilibrium geometries for several adsorption sites have been determined by the ASED-MO method. The results show that the center superposition site (hcp) with the CN axis parallel to the surface and to the [ 11 2 ] direction is the most stable site and the optimized height is equal to 1.51 Å. We also calculated the electronic structures with the DV-Xα method. In the total DOS curves for the equilibrium adsorption geometry, four peaks located at 8.77, 7.25, 6.02 and 5.09 eV below E F are found. They are determined to be the 4σ(a′), 1π(a′), 1π(a″ and 5σ(a′) levels respectively. The total charge transferred from the substrate to the CN is about 0.51 electrons, which is mainly in to the 5σ orbital.

Molecular cluster calculations for the analysis of N 2 chemisorption on nickel surfaces

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of nitrogen on nickel surfaces have been performed. The clusters are Ni 5N 2, Ni 7N 2 and Ni 9N 2 for N 2 on Ni(100), (111) and (110) surfaces respectively, with the N-N distance equal to the free molecular value. The theoretical results for the binding energy show that the atop chemisorption on Ni(110) is stronger than on the other two surfaces. The optimized Ni-N bond length is 1.80 Å on Ni(110) and 1.85 Å on the others. In the ground-state valence levels, 4ĩσ is the main bonding orbital. In the total DOS and difference curve for Ni 9N 2, the 4ĩσ and 1ĩπ + 5ĩσ peaks are located at 11.9 and 8.1 eV below E F, in good agreement with the UPS results. The N 2 2ĩπ induced small peaks are around 2.5 eV below E F, which is related to the tendency of N 2 dissociation. The charge transfer in N 2 chemisorption is done by a donation-backdonation scheme.

Molecular cluster calculations for the analysis of N 2 chemisorption on nickel surfaces

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of nitrogen on nickel surfaces have been performed. The clusters are Ni 5N 2, Ni 7N 2 and Ni 9N 2 for N 2 on Ni(100), (111) and (110) surfaces respectively, with the N-N distance equal to the free molecular value. The theoretical results for the binding energy show that the atop chemisorption on Ni(110) is stronger than on the other two surfaces. The optimized Ni-N bond length is 1.80 Å on Ni(110) and 1.85 Å on the others. In the ground-state valence levels, 4 \\ ̃ gs is the main bonding orbital. In the total DOS and difference curve for Ni 9N 2, the 4 \\ ̃ gs and 1 \\ ̃ gpu + 5 \\ ̃ gs peaks are located at 11.9 and 8.1 eV below E F, in good agreement with the UPS results. The N 2 2 \\ ̃ gp induced small peaks are around 2.5 eV below E F, which is related to the tendency of N 2 dissociation. The charge transfer in N 2 chemisorption is done by a donation-backdonation scheme.

Electronic densities of states of semi-infinite disordered chains: Comparisons of exact and analytic calculations.

Results of exact and analytic calculations of the electronic densities of states (DOS's) associated with semi-infinite substitutionally disordered chains are presented using the exact position-space renormalization-group (PSRG) method, the augmented-space (AS) formalism, and the embedded-cluster method (ECM). In addition to total DOS's, the PSRG method allows the calculation of exact partial DOS's associated with local atomic configurations in a disordered material. Comparisons with the exact results indicate that as in the case of infinite materials the ECM provides a reliable method for the calculation of single-particle properties, such as the DOS, of semi-infinite systems. Furthermore, the ECM is found to be much more accurate than the AS formalism, especially in the case of concentrated substitutionally disordered alloys.